Measurements of Rudder and Wheel Torque at Speed
by , s/v Beatrix (KP44 #276, 1980)
Although it is possible to calculate angular rudder torque from the square feet of the rudder, I felt an empirical method was more accurate. This data was gathered aboard Beatrix by putting an automotive torque wrench on the steering wheel nut. "Spokes" were translated to degrees of wheel turn and rudder angles. This data was used in evaluating the stroke (or "throw length") and power required when using a linear actuator for a below-decks steering system. The experiment was done at 6 knots. The hull speed data is an extrapolation.
Most actuators have a stroke of about 12".
The Octopus unit used aboard Beatrix has a stroke of 300mm (or
11.8" ) and 400kg (or 882 pounds) of thrust. Once the data below was
gathered I could determine what rudder angle and tiller length was best for this
unit. A longer tiller arm gives more leverage, but restricts the allowable
rudder angle. Refer to the table below. Using a 7" tiller will
allow full rudder travel, but raises the thrust (and power) required for turns
at speed. The
intersection of the 10" Tiller column with the 37.5 degree rudder angle row
in the spreadsheet shows the required thrust to be 660 pounds, about 75% of the
Octopus' rated thrust. The 10" tiller is more appropriate for the KP44, but requires reducing the
factory rudder limit from 45º to 37.5º on either side of center.
Reducing the rudder angle also improves safety in following seas or when backing
down hard. I found no noticeable handling
differences docking the boat with the lower rudder angle. Also keep in mind that an
electric autopilot should not normally be required to "hard-over" the
helm at hull speed.
To see how the rudder angle restriction was done and for more information on the Octopus and the installation aboard Beatrix, click here.
|Wheel to Rudder Turn Ratio||11|
|Rudder Quadrant Degrees||90|
|Steering Wheel Diameter||30|
|Speed during test (knots)||6|
|Hull Speed Correction||1.927469|
|AT 6 KNOTS||AT HULL SPEED|
|6-Spoke Wheel Turns||Wheel Degrees||Rudder Angle in Degrees||Measured Wheel Torque in Ft-Lb||Rudder Post Torque Ft-Lb||10" Tiller (pounds)||7" Tiller (pounds)||Rudder Post Torque Ft-Lb||10" Tiller (pounds)||7" Tiller (pounds)|
|1 turn 2.25 spokes||495||45.0||78||858||1030||1471||1654||1985||2835||Hard Over Limit|
|1 turn 1.79 spokes||468||42.5|
|1 turn 1.33 spokes||440||40.0|
|1 turn 0.88 spokes||413||37.5||50||550||660||943||1060||1272||1817||New Hard Over Limit|
|1 turn 0.42 spokes||385||35.0|